To nit-pick on a creative project that is not your own, usually focusing on minor details that eventually stack up and change the look and feel of the piece. Very unnerving to the creative in charge of the project, especially if the pixel fucker is standing right behind the creative during the production.

Account Exec: Hey...you finish editing that spot yesterday?
Editor: Hell no! I thought we were going to be done by lunch time, but the stupid client pixel fucked it until midnight. He'll be here in a hour for more.